Light Emitting Diodes (LEDs) have many advantages over conventional light sources, such as incandescent, halogen and fluorescent lamps. These advantages include longer operating life, lower power consumption, and smaller size. Consequently, conventional light sources are increasingly being replaced with LEDs in traditional lighting applications. As an example, LEDs are currently being used in flashlights, camera flashes, traffic signal lights, automotive taillights and display devices.
At present, LED dice can be attached to an LED package using one of two methods. A first method is referred to as eutectic bonding. A second method employs Dice Attach (DA) glue or adhesive. Two different type of DA glues are commonly used: metal-filled glue and transparent/non-conductive glue.
Nowadays, transparent/non-conductive glue is becoming increasingly popular as compared to eutectic bonding due to the low startup costs associated therewith and due to the easy handling. Also, transparent/non-conductive glue provides a better brightness as compared to metal-filled glues, especially when used with transparent LED dice.
With respect to thermal conductivity between dice and package, eutectic bonding provides the best results with the lowest thermal conductivity, followed by metal-filled glue, and finally transparent/non-conductive glue. Therefore, the use of transparent/non-conductive glue is usually not recommended in mid- or high-power products because of the inconsistency of thermal conductivity.
Transparent/non-conductive glue is generally not recommended for mid- or high-power light emitting products because thermal conductivity from the LED dice to the bonding pad is determined by the size of the gap between the LED dice and the bonding pad (a characteristic commonly referred to as Bond Line Thickness (BLT)). The smaller the gap, the better the thermal conductivity.
In industrial standard practices, to obtain optimal thermal conductivity with transparent/non-conductive glue, the gap thickness is desired to be less than 1 um. However, currently available processes for attaching a LED dice to a bonding pad do not reliably control the gap to be less than 1 um. This lack of control over the gap thickness is due to many factors. One factor in particular is that the low viscosity of the transparent/non-conductive glue causes the LED dice to be easily displaced (e.g., moved under application of an outside force, migrated under the LED's own weight, etc.) before the glue is set or cured. Accordingly, it is highly likely that utilization of a transparent/non-conductive glue will result in a gap between the LED dice and bonding pad that is greater than 1 um, which means that the heat generated by the LED dice will not be sufficiently carried away from the LED dice, which means that if the LED dice is operating as a mid- or high-power LED, then the chances of the LED dice failing are greatly increased.
In other words, there are significant challenges with using transparent/non-conductive glue to achieve low thermal conductivity (e.g., uniform gap thickness between the LED dice and bonding pad).